Support Foreign Markup in APP requests

Description

At the moment, this feature request is more of a placeholder to solicit design feedback (Joseph Scott suggested I open it for this purpose).

Foreign Markup in Atom is arbitrary XML in a non-Atom namespace that is included in an Atom entry. It could be stored in an additional database column, be a special form of an attachment, or could build upon WP's support for custom fields.

The notion of custom fields is attractive as it would enable users to directly manipulate this data from the GUI. It would also provide a means for the data that the user already enters to appear in their feed. While these are both good, we need to be prepared for the possibility that there will be custom data that the user won't want to put in their feed, and foreign markup that the user won't want to see on their screen (as it is arbitrary XML after all...).

Suggestions welcome. If a design is agreed upon, I will help with the development and testing. I would like to request that this goes into 2.4.